Const
Means that the identifier can’t be reassigned

const name = 'Shane'

 
Let
Means that the identifier may be reassigned. Such as a counter in a loop, or a value swap in an algorithm. It also signals that the variable will be used only in the block it’s defined in, which is not always the entire containing function.

let age = 23

N.B. you have to initialize your const and let identifiers before using them as typeof won’t work.
 
Arrow functions
Shorter syntax for writing function expressions

// ES5
var nameIs = function(name) {
   return name
}

// one parameter
var nameIs = name => {return name}

// More than one parameter
var nameIs = (x, y) => {return name}

// No parameter
var nameIs = () => {return name}

With array methods

var names = [
  {
    name: 'shane',
    age: 13
  },
  {
    name: 'paul',
    age: 21
  }, 
];

var sorted = names.map(
  names => names.age
)

console.log(sorted);

 
Refs
ES6 Arrow functions – Sitepoint
JavaScript ES6+: var, let, or const – Eric Elliot

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